Axenically cultured Giardia lamblia trophozoites (2 × 10⁵) were inoculated by gavage into suckling mice. All mice three, seven, and 14 days old became infected, with the peak trophozoite count in the small bowel of 5.3 ± 1.1 × 10⁶ occurring seven days after inoculation of three-day-old mice. In addition to infecting each of 65 challenged mice, G. lamblia was able to complete its life cycle by encysting and, as cysts, to infect previously unchallenged animals. Control mice given culture medium alone, mice challenged with trophozoites when older than 14 days of age, and mothers of infected mice did not become infected. There was no difference in body weight between infected mice and control mice, nor were histopathologic changes in the small bowel noted at the time of peak infection. Infection was cleared in all mice by 17–21 days. Thus, axenically cultured G. lamblia trophozoites are capable of infecting and completing their life cycle in suckling mice. The susceptibility of mice to infection is remarkably age specific, and the infection is apparently without adverse effects on the murine host.